Optical disk devices utilize laser light for recording data onto and sensing data from storage media. To write data on optical media, the laser is operated at a relatively high, rapidly switchable power level in order that the medium can be altered in accordance with the input data stream. In reading the data back, the laser power level is controlled at a lower level so that the medium is not altered by the laser beam, but the reflected light indicates the presence or absence of medium alterations caused by the input data stream. In erasing data, the light source operates continuously at a power level lower than the write level. In a typical optical disk device, the laser power at the disk is modulated between a high and a low level for data recording, typically 15-20 mW and 0.5 mW, and is 2 mW for reading and 8 mW for erasing.
Semiconductor diode lasers are presently the light source of preference in optical disk systems. They are lightweight, efficient in electrical-to-optical power conversion, and can be amplitude-modulated by control of the injection current. Because of losses in beam collimating, circularizing, and focussing elements, the path efficiency for coupling the generated light to the disk is about 50%. Thus the power requirements of the light source-are twice those required at the disk, and 50 mW diode lasers are typically employed.
The stability of the laser emission is a major factor in determining the signal-to-noise of data retrieval. Diode lasers are subject to mode hops between Fabry-Perot cavity resonances, particularly when operated continuous wave (CW) at low power as in the data reading process. Light reflected from the optical disk directly back to the laser exacerbates the instability. In systems where data is retrieved by intensity contrasts or phase shifts, polarizing beamsplitters and quarter-wave plates can be positioned in the beam path to minimize the return of reflected light to the laser. However, in magneto-optic (MO) systems feedback cannot be eliminated with passive optical components because the signal is detected as elliptical polarization produced at the disk surface by Kerr rotation of incident, linearly-polarized, light. Moreover, feedback noise is particularly serious in MO systems because of the limited signal level available.
Several methods have been proposed to reduce laser feedback. High frequency modulation of the laser can be used to reduce the noise, but at the cost of increased electronic circuitry, radio frequency interference, and laser power transients. Placing a beam attenuator, such as a filter or polarizer, with transmittance T in the optical path between the laser source and the optical disk attenuates 1-T.sup.2 of the reflected beam which returns to the laser cavity (U.S. Pat. No. 4,337,535 and U.S. Pat. No. 4,835,761). However, it also attenuates 1-T of the beam which reaches the disk and the generated laser power must be correspondingly increased.